It is generally known that a large magnetizing inrush current flows when non-load magnetizing is performed by power-on while a transformer core has a residual magnetic flux. The magnitude of this magnetizing inrush current is several times as large as the rated load current of the transformer. The system voltage fluctuates if a large magnetizing inrush current like this flows. If this voltage fluctuation is large, it may have an influence on users.
As a method of suppressing this magnetizing inrush current, a method is known in which when turning on a directly grounded three-phase transformer by using three single-phase circuit-breakers, one arbitrary phase is first closed, and then two remaining phases are closed, thereby suppressing a magnetizing inrush current.
There is also a known method in which a circuit-breaker of a reference phase is closed at a timing at which a prospective magnetic flux and residual magnetic flux of the reference phase match, and then two remaining circuit-breakers are closed with a delay so as to minimize the difference between a prospective magnetic flux and residual magnetic flux of each of other phases.
There is, however, a circuit-breaker in which a capacitor is connected in parallel between the poles in order to facilitate current interruption. When shutting down a transformer by this circuit-breaker in which the capacitor is connected between the poles, it is difficult for the above-described methods to suppress a magnetizing inrush current for the following reason.
When shutting down a transformer by the circuit-breaker in which a capacitor is connected between the poles, an AC voltage having a small amplitude appears at a transformer terminal after shutdown due to the inter-pole capacitor. This AC voltage is a power supply voltage divided by the inter-pole capacitor and the stray capacitance of the transformer. In this state, the residual magnetic flux of the transformer is obtained by superposing a component of this small-amplitude AC voltage on a DC component.
When the residual magnetic flux is thus obtained by superposing the AC component on the DC component, it is difficult to obtain the intersection of the residual magnetic flux and prospective magnetic flux as described above. This is so because the DC component of the residual magnetic flux changes in accordance with the breaking phase of the circuit-breaker, and the magnitude of the AC component changes in accordance with the capacitance of the inter-pole capacitor.
Under the circumstances, it is desired to provide a magnetizing inrush current suppressing device capable of suppressing a transformer magnetizing inrush current to be generated when powering on a transformer by using a circuit-breaker in which a capacitor is connected between the poles.